Electric screw actuator system

ABSTRACT

An electric screw actuator system, e.g. for an electrically driven and controlled steering device, comprises a screw actuator, an electric motor ( 2 ) engaging the screw actuator through a reduction gear mechanism ( 3 ), electronic operating means for operating the screw actuator electrically, and a mechanical back-up system for operating the screw actuator in the absence of electric power e.g. in the case of an electric power failure of in the case the electric power is switched off, maintenance, garage parking etc. The mechanical back-up system comprises a manually controllable operation member ( 4 ) and an electromagnetic coupling ( 6 ), said coupling disengaging the rotation member from the screw actuator in the case of nominal operation and availabitity of electric power, and engaging the rotation member with the screw actuator in the case of electric power failure.

[0001] The invention is related to the field of electric screw actuatorsystems which are to be applied as the main drive for all kinds oftasks, such as the electric drive and control of the steering device ofa vehicle, the brake device thereof etc. A major point of concern inthis field is the degree of reliability of the electric componentsthereof, including the supply of electric current which is necessary forpowering the system.

[0002] Although the reliability has meanwhile been raised to a levelwhich is accepted in most fields of technology, e.g. the aircraftindustry, and the automotive industry, nevertheless several reasonsstill exist to apply a mechanical back-up system. Such mechanicalback-up system might not always be necessary for reasons of lack ofreliability, but other reasons, such as a feeling of confidence by thecar driver, (long term) garage parking etc. may make it desirable toincorporate a mechanical back-up.

[0003] The invention aims at providing such electric screw actuatorsystem, e.g. for an electrically driven and controlled steering device.Said system comprises a screw actuator, an electric motor engaging thescrew actuator through a reduction gear mechanism, electronic operatingmeans for operating the screw actuator electrically, and a mechanicalback-up system for operating the screw actuator in the absence ofelectric power, e.g. in the case of an electric power failure or in thecase the electric power is switched off, maintenance, garage parkingetc., said mechanical back-up system comprising a manually controllableoperation member and an electromagnetic coupling, said couplingdisengaging the rotation member from the screw actuator in the case ofnominal operation and availability of electric power, and engaging therotation member with the screw actuator in the case of absence of electpower.

[0004] In the electric screw actuator system according to the invention,an automatic switch takes place from electric operation towards manual,full mechanic operation when, for whatever reason, no electric currentis available. Under normal conditions however, full electric drive andcontrol takes place, for instance through control buttons, a joysticketc.

[0005] Although the mechanical back-up is primarily intended to providerelief under emergency conditions, additional “normal” or desiredfunctions can be attributed thereto as well. In the case of automotiveapplications for instance it may be desirable to apply additionalmechanical actuation of the park brake for long term braking of thevehicle, such as uninterrupted parking for several weeks or months. Insuch case, the electric battery would gradually loose most of its energywhen electric drive of the parking brake would be applied.

[0006] The electric motor is usually a relatively high revolution motor,which necessitates the application of the reduction gear mechanism. Onthe other hand, the manually controllable operation member, such as asteer, a stick etc., usually only provides a limited displacement orrotation. Thus, according to the invention, an auxiliary gear mechanismis provided for obtaining the necessary number of revolutions of the nutor screw so as to obtain the required displacement. Said auxiliary gearmechanism is an accelerating gear mechanism.

[0007] Preferably, the reduction gear mechanism and the auxiliary gearmechanism are integrated. In that case, the reduction gear mechanismcomprises a sun gear wheel which is driveable by the electric motor, afixed ring gear wheel connected to one of the nut and the screw,satellite gear wheels which engage the sun gear wheel and the ring gearwheel, and a carrier onto which the satellite gear wheels are rotatablymounted, said carrier engaging the other of the nut and the screw.

[0008] In a compact embodiment, the nut of the screw actuator isconnected to a housing, said housing furthermore carrying the fixed ringgear wheel(s) which is (are) concentric with respect to the nut and thescrew, as well as the electromagnetic coupling.

[0009] Furthermore, the axis of the coupling can be transverse withrespect to the axis of the screw actuator, in which case the couplingengages the auxiliary gear mechanism through bevel gears.

[0010] The mechanical back-up system may comprise a flexible cable drivewhich at one end is connected to the coupling, and which at the otherend is provided with a manually operated lever means, e.g. a steer,joystick or the like. An additional gear acceleration unit may also belocated near the joystick.

[0011] The manually operated lever means has a swivel point which isfixed with respect to the screw actuator, the flexible cable drive beingconnected to the lever means at the swivel point thereof,electromagnetic immobilisation means being provided for preventingrotation of the lever means in the case of nominal operation andavailability of electric power, and for allowing rotation of the levermeans in the case of absence of electric power.

[0012] Preferably, the lever means carry the electronic operating meansfor electrically operating the screw actuator, like existingdrive-by-wire system. This electronic operating means may for instancebe a joystick which is directly mounted to the swivel point, or at adistance therefrom at the free end of the lever means.

[0013] A sensor may be provided for monitoring and controlling adisplacement and/or a rotation of e.g. the nut and/or the screw inclosed loop configuration with the electric motor, the electronicoperating means, and an electronic control unit.

[0014] As mentioned, the system according to the invention may beapplied to steer a vehicle. Thus, the invention is also related to anelectromechanical steering device, comprising an electric screw actuatorsystem as described before and a joystick having at least one forcesensor and/or at least one displacement or rotation sensor for steering,and/or braking, and or accelerating action.

[0015] The invention will now be described further with reference to anelectromechanical steering device according to the invention as shown inthe drawings.

[0016]FIG. 1 shows the general layout of a vehicle provided with anelectric steering and brake device according to the invention.

[0017]FIG. 2 shows the general layout of the electric steering deviceaccording to the invention.

[0018]FIG. 3 shows a cross-section of the screw actuator.

[0019] The vehicle shown in FIG. 1 comprises a steering actuator 1 whichis connected to the front wheels of the vehicle, as well as a joystick26 situated near the driver's seat. Furthermore, electromechanical brakeactuators 28 are provided which control the disk brakes on all wheels ofthe vehicle, and which also may be controlled by the joystick 26.Electromechanical drum brakes may also form a part of a completeelectromechanical brake system.

[0020] The joystick 26 is connected to an electronic control unit 27,which through the driving dynamic controller 29 controls the electricmotor 2 of the screw actuator 1.

[0021] The joystick 26 also controls the electromechanical brakeactuators 28 on the basis of signals received from the wheel speedsensors 30. Furthermore, sensors 31 have been incorporated which monitorthe driving status, all sensors being connected to the sensorelectronics control device 32.

[0022] As shown in FIG. 2, the control unit 27 receives signals relatede.g. to the load exerted on the screw actuator 1, displacements and/orrotations of components thereof, through cable 33. Through cable 34, thecontrol unit 27 receives signals from the joystick 26. On the basis ofthese signals, the electric motor 2 is controlled, through cable 35, foractuating the screw actuator 1.

[0023] The joystick 26 is mounted at the free end of the lever 23, whichis rotatable around a vertical axis on the swivel point 24. Furthermore,the lever 23 can pivot about the horizontal pivot axis of the hinge 36.

[0024] By means of the immobilisation means 25, e.g. an electromagneticcoupling, the lever 23, and thereby the joystick 26 are held in anominal position under nominal conditions, that is when electric currentis available.

[0025] In the absence of electric current, e.g. when the electriccurrent is switched off or in case of an electric power failure, theelectromagnetic coupling 25 is released and by manipulating the lever 23trough the joystick 26, a rotation through swivel 24 is imparted on theflexible cable 4 which is capable to transmit a torque.

[0026] In turn, this flexible cable 4 drives the screw actuator 1 insuch a way that steering action is still possible.

[0027] Additionally, a flexible brake cable 37 is provided, which, inthe absence of an electric power, is controlled by pivoting the lever 23about the hinge 36. The flexible cable 37 is connected to the brakeactors 28, but is not shown in further detail.

[0028] The actuator applied can be executed with a rotating nut and amoving screw, or a moving nut and a rotating screw.

[0029] As shown in FIG. 3, the screw actuator 1 is accommodated in ahousing 19, which is non-rotatably fixed with respect to e.g. thechassis of a vehicle. The nut 12 is immovably fixed within the housing12. The screw 13 is rotatably and translatably supported within the nut12 by means of the balls 38, which engage correspondingly shaped screwthreads in the nut 12 and the screw 13.

[0030] The screw actuator 1 is actuated through an electric motor 2which two bevel pinion 39 drives a first reduction bevel gear 40 whichis positioned around the screw 13. The bevel gear 40 is rotatablysupported by means of bearing 41. The bevel gear 40 is integrated with asun gear wheel 8, which engages satellite gear wheels 10. The housing 1carries a fixed ring gear wheel 9, which also engages the satellite gearwheels 10.

[0031] The satellite gear wheels 10 are rotatably mounted on a carrier11, which through the spline/ball mechanism 20 is able to rotate thescrew 13.

[0032] The bevel gears 39, 40, as well as the satellite gear wheelmechanism 8-11 provide the necessary reduction of the motor rotationsfor driving the screw actuator 1.

[0033] In case of an electric power interruption, the electric motor 2is no longer able to drive the screw actuator 1. For this reason, amechanical back-up system is provided, comprising a flexible cable 4,which is connected to electromagnetic coupling 6. In case of a powerinterruption 6, this electromagnetic coupling 6 connect the flexiblecable 4 to the auxiliary drive (bevel) gear wheel 14. The auxiliarydrive gear wheel 14 engages a bevel gear 21, which surrounds the screw13. The bevel gear 21 is integrated with an auxiliary carrier 15,rotatably supported with respect to the housing by means of the bearing42.

[0034] The carrier 15 carries auxiliary satellite gear wheels 16, whichengage a fixed auxiliary ring gear wheel 17, as well as an auxiliary sungear wheel 18 integrated with the carrier 11.

[0035] By rotating the flexible cable through the lever 23 (see FIG. 2)the screw 13 can thus be rotated mechanically.

[0036] The bevel gears 14, 21 or other appropriate gear transmissionprinciples are preferably accelerating gear wheels.

1. Electric screw actuator system, e.g. for an electrically driven andcontrolled steering device, said system comprising a screw actor (1), anelectric motor (2) engaging the screw actuator (1) through a reductiongear mechanism (3), electronic operating means (26) for operating thescrew actuator (1) electrically, and a mechanical back-up system (4) foroperating the screw actor (1) in the absence of electric power e.g. inthe case of an electric power failure or in the case the electric poweris switched off, maintenance, garage pang etc., said mechanical back-upsystem (4) comprising a manually controllable operation member (5) andan electromagnetic coupling (6), said coupling (6) disengaging therotation member (5) from the screw actuator (1) in the case of nominaloperation and availability of electric power, and engaging the rotationmember (5) with the screw actuator (1) in the case of absence ofelectric power.
 2. System according to claim 1, wherein anelectromagnetic coupling (6) is connected to an auxiliary gear mechanism(7) which engages the actuator (1) in case of absence of electric power.3. System according to claim 2, wherein the reduction gear mechanism (2)and the auxiliary gear mechanism (7) are integrated.
 4. System accordingto claim 3, wherein the reduction gear mechanism (2) comprises a sungear wheel (8) which is driveable by the electric motor (2), a fixedring gear wheel (9) connected to one of the nut (12) and the screw (13),satellite gear wheels (10) which engage the sun gear wheel (8) and thering gear wheel (9), and a carrier (11) onto which the satellite gearwheels (10) are rotatably mounted, said carrier (11) engaging the otherof the nut (12) and the screw (13).
 5. System according to claim 4,wherein the coupling (6) is connected to an auxiliary drive gear wheel(14) which engages the screw actor (17).
 6. System according to claim 5,wherein the auxiliary drive gear wheel (14) engages an auxiliary carrier(15) onto which auxiliary satellite gear wheel (16) are rotatablymounted, said auxiliary satellite gear wheels (16) engaging an auxiliaryring gear wheel (17) which is fixed with respect to the other ring gearwheel (9), and an auxiliary sun gear wheel (18) connected to the othercarrier (11).
 7. System according to any of claims 2-6, wherein theauxiliary gear mechanism (7) is an accelerating gear mechanism. 8.System according to any of claim 4-7, wherein the nut (12) of the screwactuator (1) is connected to a housing (19), said housing (19)furthermore carrying the fixed ring gear wheels (5, 9, 17) which is(are) concentric with respect to the out (11) and the screw (12), aswell as the electromagnetic coupling (6).
 9. System according to claim8, wherein the screw (12) engages the carrier (11) through a spline/ballmechanism (20).
 10. System according to claim 8 or claim 9, wherein theaxis of the coupling (6) is transverse with to the axis of the screwactuator (1), and the coupling (6) engages the auxiliary gear mechanism(7) through bevel gears (14, 21).
 11. System according to any of thepreceding claims, wherein the mechanical back-up system (4) comprises aflexible cable drive (22), capable of transmission a torque, or apulling cable or a mechanical linkage, which at one end is connected tothe coupling (6), and which at the other end is provided with an handoperated lever means (23), e.g. a steer, joystick or the like. 12.System according to claim 11, wherein the manually operated lever means(23) has a swivel point (24) which is fixed with respect to the screwactuator (1), the flexible cable drive (22) being connected to the levermeans (22) at the swivel point (24) thereof, electromagneticimmobilisation means (25) being provided for preventing rotation of thelever means (23) in the case of nominal operation and availability ofelectric power, and for allowing rotation of the lever means (23) in thecase of electric power failure.
 13. System according to claim 12,wherein the lever means (23) carries said electronic operating means(26) for operating the screw actuator (1) electrically.
 14. Systemaccording to claim 13, wherein the electronic operating means comprisesa joy stick (26).
 15. System according to claim 14, wherein the joystickis at a distance from the swivel point (24).
 16. System according toclaim 14, wherein the joystick (26) is directly mounted to the swivelpoint (24), and is rotatable and/or liftable (x-y-z axis).
 17. Systemaccording to any of the preceding claims, wherein a sensor is providedfor monitoring and controlling a displacement and/or a rotation of e.g.the nut and/or the screw in closed loop configuration with the electricmotor (2), the electronic operating means (26), and an electroniccontrol unit (27).
 18. System according to any of the preceding claims,wherein the reduction gear mechanism (13) comprises an excentric gearreduction.
 19. System according to any of the preceding claims, whereinthe reduction gear mechanism (13) comprises a combination of bevelgears, and/or planetary gears, and/or excentric gears.
 20. Systemaccording to any of the preceding claims, wherein the screw actuator (1)has a rotatably supported nut (12) and a non-rotatable, translatablescrew (13).
 21. System according to any of claims 1-19, wherein thescrew actuator has a non-rotatable, translatable nut and a rotatablysupported screw.
 22. System according to claim 21, wherein the nut isconnected to or integrated with a piston.
 23. Electromechanical steeringsystem, comprising an electric screw actuator system according the oneof the claims 1-21, and comprising a joystick (26) having at least oneforce sensor and/or at least one displacement or rotation sensorsteering, and/or braking, and or accelerating action. 24.Electromechanical brake system, comprising an electric screw actuatorsystem according to one of claims 1-21.